Phase modulation techniques are used for communication systems, and such phase modulation techniques incorporate phase shift circuits of the type wherein the phase shift is adjustable over a 360 degree range.
An example of a typical adjustable phase shifter for adjustably providing a 360 degree range of phase shift to RF signals is disclosed in U.S. Pat. No. 4,395,687 issued Jul. 26, 1983 to Belohoubek and entitled, "Adjustable Phase Shifter". In the U.S. Pat. No. 4,395,687 the RF signals are divided into three equal parts, adjusting the relative amplitudes of the parts of the RF signals and combining the three parts of the RF signals at relative phase angles of 0, 120 and 240 degrees.
In U.S. Pat. No. 4,355,289 issued Oct. 19, 1982 to Beyer et. al. and entitled "Phase Shift And Amplitude Modulator", a parallel modulator for digital phase modulation is described wherein the outputs of two amplitude controllable bi-phase modulators are combined in quadrature to form the basis for the output signal; by control of the amplitude and phase--either 0 or 180 degrees--of each of the bi-phase modulators and an output signal having an arbitrary phase angle is produced.
The parallel modulator is capable of independent phase and amplitude control and employs bi-phase modulators.
Each bi-phase modulator utilizes diodes which are always turned on so as to minimize the parasitic reactances associated with the diodes, and each bi-phase modulator includes a power splitter which splits the continuous wave carrier input signal into two intermediate signals which are equal in amplitude and 180 degrees out of phase. At microwave frequency operation, each of the two intermediate signals are coupled through directional couplers to a pair of PIN diodes which are connected together at a common summing junction.
With proper adjustment of the DC bias level, both diodes will be operated fully on, and the two microwave frequency intermediate signals conducted through the diodes will exactly cancel at the summing junction--a null condition. If the bias current provided to the diode which is conducting the intermediate signal at 0 degree phase angle is reduced from the null condition current, the intermediate signal at 180 degrees will not be completely canceled at the junction, and thus the phase of the output signal will be at 180 degrees.
The output amplitude may thereafter be continuously controlled by varying the bias current of the diode conducting the 0.degree. intermediate signal.
Two bi-phase modulators of the Beyer patent may be combined in a parallel modulator configuration to yield a modulated output signal which, with proper control of the bias inputs to the bi-phase modulators, can be controlled to an arbitrary phase angle between 0.degree. and 360 degrees, and to a continuously controllable amplitude.
In U.S. Pat. No. 4,549,152 issued Oct. 22, 1985 to Kumar and entitled, "Broadband Adjustable Phase Modulation Circuit, a phase modulation circuit is described which provides at an output terminal an output signal controllably shifted from 0 to 360 degrees relative to an input signal provided at an input terminal and includes a first 90 degree coupler connected between the input terminal and second and third 90 degree couplers, four gain controllable amplifiers, fourth and fifth 90 degree couplers and an in phase combiner with its output connected to the output terminal. Two of the amplifiers are connected between the second and fourth couplers. The additional amplifiers are connected between the third and fifth couplers. The fourth and fifth couplers are connected to the inputs of the in-phase combiner. The gain through the various amplifiers is adjusted to control the amount of phase shift through the circuit.
U.S. Pat. No. 4,581,595 issued Apr. 8, 1986 to Silagi entitled "Phase Shift Network With Minimum Amplitude Ripple", describes an improved phase shift network which includes a splitter for receiving an input signal and providing outputs which are coupled through all pass networks to produce signals differing in phase by predetermined angle. The signals from the all pass networks are coupled through variable attenuators having their outputs recombined in a combiner to produce an output representing the input signal phase shifted by a predetermined angle. The phase shift may be adjusted by changing only one variable attenuator at a time to produce phase shifts of 90 degrees or greater.
U.S. Pat. No. 4,870,374 issued Sep. 26, 1989 to Harris entitled "Modulator Producing Phase Modulation By Combining Amplitude Modulated Signals", describes still another system for using the modulation of the phase of a RF carrier as means of transmitting information. The RF input is split into four quadrature components each of which is passed through individual amplitude control attenuators and then summed in a combiner to create the required phase-controlled RF signal. The resultant signal is then amplified in an RF signal amplifier chain as required to achieve the desired output level. The RF output level is sampled and routed back to an in-phase splitter the output of which is compared to the corresponding individual quadrature components of the unmodulated RF signal to generate a signal which is compared with a processor generated modulation reference signal in a control loop amplifier and the resulting error signal is applied to the individual attenuators to adjust the output amplitude of each quadrature component. The quadrature components are then combined to obtain the desired phase-modulated RF output signal.